Interdigital line traveling wave amplifier



Dec. 8, 1959 R. R. MOATS ETAL 2,916,656

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IF I G. 2 l8 z g g Ef/I/ IFIG.3 \la E'AM FORMING STRUCTURE INVENTORS. JOHN W. DAWSON, ROBERT R. MOATS and MARSHALL C. PEASE III.

BY ATT'Y Dec. 8, 1959 R. R. MOATS ETAI- 2,916,656

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INVENTORIS' JOHN W. DAWSON, ROBERT R. MOATS and MARSHALL C. PEASEJH ATT'Y United States Patent INTERDIGITAL LINE TRAVELING WAVE AMPLIFIER Robert R. Moats, Saratoga, Calif., John W. Dawson,

Williamsport, Pa., and Marshall C. Pease III, Saratoga, Calif., assignors, by mesne assignments, to Sylvania Electric Products Inc, Wilmington, DeL, a corporation of Delaware Application July 15, 1958, sesame. 748,697

4 Claims. c1. 315-45 This invention relates generally to traveling wave tubes, and more particularly to a broad-band interdigital delay line amplifier operating on forward wave principles and to Constructions of the slow wave structures, which forman integral and essential portion of said tubes, which impartto such structures characteristics which permit operation of the tube on forward wave principles.

A number of types of traveling wave tubes are known, and, depending largely upon the nature of the slow wave structure, operate on forward or backward wave principles to achieve amplification or generation of oscillations, respectively. The most common form of amplifier comprises an electron gun mounted at one end of an elongated tubular envelope and" arranged to direct a narrow electron beam toward a collector mounted at the opposite end of the envelope. Mounted between the electron gun and the collector and surrounding the path of the electron beam there is provided a slow wave structure, which usually comprises a long, slender helix having terminals at the electron gun end for supplying the high frequency signal to be amplified, and at the collector end, terminals connecting to a load. Tubes of this type are relatively-'broad-band, but are inherently low power and lowefiicien'cy devices. Currently, 100 watts is typical at S-band frequencies, and t ubes of this type have relatively low efficiency, typically to To obtain a satisfactory bandwidth with this type of tube,,the helix must be mounted with a minimum of dielectric support, and hence the helix is afheat trap, and easily burned out by interception, current, as well as by R-F currents. While theoretically the helix need not intercept any of the beam current, in practice it does, and the fraction that is intercepted increases as beam power is increased. Therefore, it appearsimpracticable to increase greatly the power handling capability of the helix-type traveling wave tube.-

Another form of amplifier tube, employing disc loaded wave guides, is capable of very high power operation, but currently available tubes of thistype have a very narrow bandwidth, of the order of 10% or less. To achieve non-dispersiveness and effective beam coupling in the TM mode of a circular wave guide, the loading discs are designed and arranged to cause the whole structure to store considerable energy. As is well known, the storage of energy in a transmission structure limits the bandwidth of the structure; hence, it appears unlikely that tubes of this typehavin'g a bandwidth suitable for many existing applications can be developed.

In short, of the currently available traveling wave tube amplifiers, the type having broad band characteristics is limited in power, and the type capable of handling reasonable amounts of power is limited in bandwidth.

It is a primary object of the present invention to provide 'a high frequency amplifier capable of high power and broadband operation.

Another object of the inventionis to provide an improved crossed-field traveling wave tube amplifier. *"Another object of the invention is to provide a slow a 2,916,656 Patented Dec. 8, 1959 wave structure for a crossed-field (M-type) tube to permit its operation on forward wave principles and thereby achieve amplification.

Still another object of the invention is to provide a high frequency amplifier having a bandwidth comparable to that of a helix type traveling wave amplifier tube and the high power capability and high efiiciency of a crossedfield oscillator.

A further object is to provide a high frequency amplifier which is considerably more rugged than helix type traveling wave amplifier tubes.

The invention will be described with reference to the accompanying drawing, in which:

Fig. 1 is a plan View showing a prior art delay line of the interdigital type;

Fig. 2 is a cross sectional view taken along line 22 of Fig. 1;

Fig. 3 is a cross sectional view through a traveling wave tube amplifier utilizing a delay line of the interdigital type in accordance with the present invention;

Fig. 4 is a top plan view of a traveling wave tube illustrating another embodiment of a delay line in accordance with the invention;

Fig. 5 is a cross sectional view taken along line 5-5 of Fig. 4; and

Fig. 6 is a diagrammatic view of the underside of the structure of Fig. 5 showing only the edges of the fingers thereof adjacent the electron beam.

Slow wave structures of the class comprehended by the present invention are generally described as backward wave structures, meaning that the fundamental space harmonic moves in the opposite direction to the flow of R-F energy in the structure. Hence, if a beam of electrons is made synchronous with the fundamental space harmonic of such a structure, it Will be moving in the opposite direction to the R-F wave with which it is interacting. Under these circumstances there is operative intrinsic feedback mechanism. The electron beam, after being modulated by the R-F wave, generates and R-F wave on the line which moves back towards the source of the electron beam, and then modulates the beam. Because of this feedback mechanism, backward wave interaction is most generally used in an oscillator-type tube. When used as an amplifier, it acts as a regenerative amplifier, finding application in a voltage tunable amplifier of limited bandwidth.

As discussed in the book entitled Traveling Wave Tubes, by I. R. Pierce (D. Van Nostrand, 1950'), pages 4978 and 157-159, a delay structure is forward or backward depending upon whether the velocity of the fundamental space harmonic is in the same direction or in the opposite direction, respectively, to the flow of R-F energy on the structure. The fundamental space harmonic is that space harmonic which has the highest velocity.

Traveling wave tubes of the crossed-field type (M- type) usually employ a so-called interdigital slow wave structure; viz., a delay line constituted by two comb-like members the fingers of. which interleave. As shown in Figs. 1 and 2, such a line consists of two parallel supports or rails 10 and 12 to which are secured interleaved fingers 14- and 16, normally formed of metal bars or plates. In a traveling Wave tube of the kind referred to, such line is disposed parallel to a plate 13, normally called a sole, and an electron beam is injected into the space 20 between the edges of the teeth 14 and 16 and the plate 1-8. The fingers 1'4 and I6 are normally maintained at a potential positive with-respect toplate 18' to produce an electric field across the space 20', and a mag:- net is provided to produce a magnetic field the lines of force of which are perpendicular both to the beam and 3 to the electric field, as indicated by the crossed circle marked H.

The interdigital structure illustrated in Figs. 1 and 2 is a backward-wave type; that is, it is an intrinsically dispersive structure in which the phase velocity is a rapid function of frequency. As such, it is suitable for a voltage tunable oscillator or amplifier, but is unsatisfactory for a broad band amplifier. The structure is, however, very suitable for high power operation. The fact that the fingers 14 and 16 are attached by a metallic connection to the supports 10 and 14, which can be liquid or air cooled, alfords a good thermal dissipation capability. This construction also results in a strong, rigid and generally rugged assembly which is considerably less sensitive to damage due to shock and vibration than is the helix tube. Since it is geometrically flat, the line is conveniently combined with the sole electrode, to establish a crossed-electric field, and it is relatively easy to produce the crossed magnetic field necessary for M- type operation. Tubes of this type are known to have high power handling capability and high eificiency, but are narrow band because of their backward wave characteristic. The interdigital structure is also Well suited for use in the O-type tube and serves to greatly increase its thermal dissipation capability as compared to the helix tube.

The backward-wave nature of the interdigital line of Figs. 1 and Z is a consequence of the phase reversal of the interaction between the electron beam and successive gaps of the line. Consider, for example, a wave traveling along a serpentine path of length L in Fig. 1 with the velocity of light, 0. The wave travels from A to B, the distance 12, in a time L/c. An electron coupling to such a wave must go from A to B, not in the time L/c, but rather in a time L/c+(2m+l)/2f, where m is any integer, positive or negative, because the sense of the electric field is reversed as the wave has traveled around the corner. Hence the electron transit time must differ from the wave transit time by an odd number of half cycles. It can readily be shown that under this coupling condition, the delay ratios of the various space harmonics seen by the electron are related by where c/v is the delay ratio of the mth space harmonic, v; is the phase velocity along the folded path, m is the mode number, and A is the free space wavelength. Interaction between electrons and the radio frequency wave is most eflective when v is greatest in magnitude, corresponding to m=1 and to a'negative value of c/v This indicates that the phase velocity is opposite to the flow of energy, and the mode of interaction between the wave and the electrons is therefore called backward. It will be seen from Eq. 1 that v varies, in general, with 7., indicating that all modes are generally dispersive. In theory, and to some extent in practice, this dispersion can be reduced or even eliminated by arranging the circuit so that v varies with in such a way as to compensate for the variation of the second term, thus making the mode non-dispersive. However, this can be done precisely only at a single frequency. The mode will therefore be non-dispersive only in the immediate neighborhood of the designed frequency. The resultant tube will still be narrow banded, although not as narrow banded as a backward-wave amplifier normally is.

To obtain true broadbandedness, it is necessary to introduce a spectrum of space harmonics whose delay ratios are related as which is typical of structures such as the helix in which there is no phase reversal of the interaction. In this case, for n=0, c/v is independent of A. providing /1 is, and the phase velocity is constant. Hence the beam can be made synchronous with this wave for all frequencies (except that c/v, does vary for frequencies near the low frequency cut-oif). Thus, a structure having these delay ratios is non-dispersive and amplification occurs over the full range of frequencies without requiring adjustment of the beam velocity.

The present invention comprehends a method of achieving the delay ratios expressed in Eq. 2 by varying the coupling of successive fingers of the interdigital line with the electron beam. Referring to the cross sectional view of Fig. 3 (which in plan would be essentially identical to Fig. l), the fingers 14' are attached to a siderail or support 10', as in Fig. l, and the fingers 16' extend from another siderail, corresponding to support 12 in Fig. l, the two supports being substantially parallel. The fingers 14' and 16' are evenly distributed throughout the length of the slow wave structure, fingers 16', however, being narrower in the vertical direction than fingers 14' so as not to extend as far toward the region of the electron beam. The fingers 16' are therefore, in effect, decoupled from the electron beam, shown in Fig. 3 as being directed from a beam forming structure 22 toward collector 24. Consequently, the radio frequency fields which interact with electrons of the beam are substantially a function only of the potentials on fingers 14'. It is seen that there is no reversal in the sense of the electric field between successive fingers 14 as occurs in the conventional interdigital line of Figs. 1 and 2. Since the environment of each finger, neglecting beam loading, is the same as every other finger, the impedance will be constant along the line, and within the frequency range where operation is undisturbed by the cut-off conditions, the resultant n=0 mode is non-dispersive. Thus, the effective path length for the wave is independent of frequency, making v also independent of frequency and making the amplifier broadband.

Figs. 4 and 5 illustrate another embodiment .of an interdigital line having substantially uniform impedance throughout its length, and affording forward-wave operation in a crossed-field type of tube. In this case, a plurality of fingers 26 are mounted on one siderail or support 28, and a like plurality of fingers 30 are supported on parallel siderail 32 and interleaved with fingers 26. The fingers 26 and 30 are preferably formed of metal strips of rectangular cross-section, but instead of being oriented normal to the electron beam as in the conventional interdigital line, the fingers 30 are inclined in one direction relative to the direction of electron flow, and fingers 26 are similarly inclined in the other direction. The upper and lower edges of fingers 26 and 30 are shaped to provide a gap 34 between the adjacent lower ends of fingers 2 6 and 30 of substantially rectangular cross section, and a gap 36 of similar cross-section between the adjacent upper ends of successive fingers. Also, the fingers are formed with flats 26a and 30a at the upper edge, and 26b and 30b at the lower edge, disposed substantially parallel to the direction of electron beam flow.

As in the case of the conventional interdigital line, the signal to be amplified is coupled to the electron gun end of the structure and is propagated along a serpentine path from A to B. It will be seen that the wave is closely coupled to the beam at gaps 34 but is effectively decoupled from the beam at gaps 36. In other words, the surfaces 26b and 30b, of the fingers presented to the electron beam, are of alternating pitch, and consequently the isolated effect of each finger is unsymmetrical and has an odd partition; i.e., there is no rcversal in the sense of the electric field between successive gaps 34. This is illustrated in Fig. 6, which shows only the surfaces 26b and 30b of the fingers 26 and 30 as viewed from the underside. Basically these surfaces are the only portions of the fingers that are involved in the interaction with the electron beam, and as shown present alternately small and large spacings to the electron beam. lyjthe gaps 34 eiiter the interaction region (gaps 36 are efiec'tively decoupled), the structural arrangement of the fingers, including gaps 3 6, insuring that the electric fields at successive gaps 34 are of the same sense. The width'of .ga s 34 andf36 between successive fingers .is constant, however, with the consequence that the propagation function does not have a frequency dependence, andthe n= mode is nondispersive except in the neighborhood of the cut-off frequencies of the line. j 4 g It is of course underst: d that ina traveling wave tube utilizing the line of Fig. 3 or Fig. 4, the various elements are mounted in a vacuum-tight casing (not shown) which may be closed at the right end by the collector 24. The ultra-high frequency input energy to be amplified is coupled to the electron gun end of the line in accordance with known techniques, for example, a coaxial line connected to the middle of the first finger 30, and an output coaxial line may be coupled to the collector end of the interdigital line in the same manner. An electron beam, designated by the arrows, projected from a suitable electron gun and beam-forming structure 22 is injected between the sole electrode 18, which may be maintained at a high negative potential by the source 38, and the line which may be connected to ground. The difference in potential between the line and the sole 18 creates in the space traversed by the beam electrostatic lines of force which are parallel to the plane of the drawing in Figs. 3 and and at right angles to the trajectory of the beam. A magnet (not shown) produces a magnetic field, designated H, the lines of force of which are perpendicular both to the beam and to the electric field. Like the line described in connection with Figs. 1 and 2, the side rails or supports for the interdigital structure of Figs. 3 and 4 may be air or liquid cooled so as to keep the fingers cool to permit the use of high beam current and attendant high power operation.

Although the invention has been described as embodied in crossed-field type of tube, it is also applicable to O-type tubes wherein the interaction of the slow wave and the electron beam takes place in a region substantially free from a steady magnetic field. In this type, the magnetic field, if any, is parallel to the flow of electrons and serves to prevent dispersion of the beam by mutual repulsion of charges.

The principles of the invention herein advanced may also be extended to structures other than interdigital types. A delay line consisting of any array of elements in which o, the phase shift per section in terms of energy propagation, is small (less than 90) but in which coupling between successive elements is such as to reverse the phase, is a backward wave type of structure. The spectrum of space harmonics corresponds to an apparent phase difierence per section of p+(2n-+l)1r, where n, as before, is any integer, positive or negative or zero. The space harmonics present correspond to Eq. 1, with =27rL/)\- The principal wave (least magnitude of phase shift per section) corresponds to n=1 and is backward. An interdigital line is one example of such a structure. If alternate elements are coupled more strongly to the electron beam than the others, the phase diiference per pair of elements is 24 +2m1r where m is any positive or negative integer or zero. Inspection indicates that even values of m in the latter expression correspond to the addition of a set of space harmonics which were not present where each element was coupled the same as the others. The spectrum of space harmonics now corresponds to Eq. 2. If 4 is directly proportional to frequency, the phase velocity of the m =0 component is constant, and a broad band amplifier is possible.

From the foregoing it is seen that applicants have provided a slow wave structure which when utilized in a traveling wave tube of the crossed-field type, results in a device which operates on forward wave principles to achieve broad band, high power operation. The line structure is analogous to t hat of a helix in respect -'to bandwidth characteristics, and its application to a crossfield device provides the high power handling capability and high efiiciency. Also, in comparison to a helix typetube, a considerably more rugged and mechanically durable tribe with respect to inertial forces is achieved.

Further modifications of the inventive concepts 'disclosed herein may now become apparent to those skilled in this art. It will, be understood therefore that thescope of the present invention is to be regarded as subject only to those limitations of the appended claims.

What is claimed is:

l. A traveling wave tube of the type including an anode in the form of a delay line, means for propagating a radio frequency wave along said delay line, a negative electrode parallel to said delay line defining therewith an interaction space and whereby an electric field is provided between the delay line and the negative electrode, means for providing within said tube a magnetic field normal to said electric field, means for projecting an electron beam normally to both said fields, and means for collecting said beam, said delay line comprising first and second spaced apart supports disposed parallel to one another and to said electron beam, a series of similar parallel interdigitally arranged elongated fingers alternately supported at one end by said first and second supports with the unsupported end of said fingers spaced from the opposite support, the edges of said fingers nearest said electron beam lying in a common plane parallel to said negative electrode, the fingers supported by said first support being oriented with their long cross-sectional dimension angularly displaced in one direction from a line normal to said negative electrode and the fingers supported by said second support being oriented with their long cross-sectional dimension similarly angularly displaced in the other direction from said line to define gaps between adjacent fingers alternately in said common plane and in a plane further removed from said electron beam than said common plane.

2. A traveling wave tube of the type including an anode in the form of a delay line, means for propagating a radio frequency wave along said delay line, a negative electrode parallel to said delay line defining therewith an interaction space and whereby an electric field is provided between the delay line and the negative electrode, means for providing within said tube a magnetic field normal to said electric field, means for projecting an electron beam normally to both said fields, and means for collecting said beam, said delay line comprising first and second spaced apart supports disposed parallel to one another and to said electron beam, a series of similar parallel interdigitally arranged elongated fingers alternately supported at one end by said first and second supports with the unsupported end of said fingers spaced from the opposite support, the edges of said fingers nearest said electron beam lying in a common plane parallel to said negative electrode, and spaced apart to define a first series of gaps, the edges of said fingers remote from said electron beam lying in a common plane and defining a second series of gaps, the fingers on said support being oriented relative to the fingers on the second support such that the gaps of said first and second series defined by adjacent fingers have different widths and alternate gaps of the first series are wider than the other gaps of the same series.

3. A traveling wave tube of the type including an anode in the form of a delay line, means for propagating an ultra-high frequency wave along said delay line, a negative electrode parallel tosaid delay line defining therewith an interaction space and whereby an electric field is provided between the delay line and the negative electrode, means for providing within said tube a magnetic field normal to said electric field, means for '7 projecting an electron beam normally to both said fields, and means for collectingsaid beam, said delay line comprising first and second spaced apart supports disposed parallel to one another and to said electron beam, a series of flat, parallel interdigitally arranged fingers alternately supported at one end by said first and second supports, successive fingers being equally but oppositely angularly displaced from a line normal to said negative electrode to define gaps between the edges of adjacent fingers alternately in a plane adjacent to said electron beam and in a plane removed from said electron beam whereby only alternate ones of said gaps are effectively axis and which is substantially non-dispersive over a Wide frequency range, said lin e comprising two symmetrical comb-like members having alternately interleaved flat fingers, successive ones of said fingers being equally but oppositely angularly displaced from a line normal to said axis to define gaps between the edges of adjacent fingers alternately in parallel planes displaced from each other.

References Cited in the file of this patent FOREIGN PATENTS 699,890 Great Britain Nov. 18, 1953 

